Phloem sap is a nutrient-rich resource that is of less chemical defense. However, the phloem sap is not widely exploited because it is not easily accessible, lack of essential amino acids and has a high osmotic potential (i.e. too concentrated) relative to the insect bodies (Corlett, 2009). Hemiptera, including aphids, is the major insect group that consumes the phloem sap, so-called sap-suckers. They have specialized mouthparts and excrete the excess sugar as 'honeydew'.
The honeydew excreted by the Hemiptera generally contains more amino acids as the sap being processed by the symbiotic micro-organisms inside the Hemiptera bodies (Corlett, 2009). The honeydew also has lower osmotic potential so it serves as a better food sources to other insects (Corlett, 2009). Ants have established an obvious mutualism with aphids as they harvest the honeydew whilst protecting the aphids from their predators, parasites and pathogens in the meantime.
I have observed this interesting mutualism on the plant Goniothalamus tapisoides where the whole plant individual was covered with these aggressive weaver ants. It is interesting to know if a 'triple alliance' is present as the plant is also benefited as the ants protect the plant from other herbivores, which can probably compensate the loss of phloem sap (Moog et al., 2005).
This ant is guarding the aphids on the inflorescence and waiting for the honeydew production.
References:
Corlett, R.T. 2009. The ecology of tropical East Asia. Oxford University Press, Oxford, UK.
Moog, J., Saw, L.G., Hashim, R. and Maschwitz, U. 2005. The triple alliance: how a plant-ant, living in an ant-plant, acquires the third partner, a scale insect. Insectes Sociaux 52: 169-176.
